Schedule of Income subject to domestic and foreign taxation (Detail) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	[1]	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[2]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	[3]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Abstract]
Income Before Tax, Domestic												$ 492.6	$ 1,289.0	$ 1,215.8
Income Before Tax, Foreign												135.2	115.0	131.9
Income Before Income Tax	$ 309.7	$ (377.8)		$ 354.8	$ 341.1	$ 345.3		$ 366.8	$ 361.6	$ 330.3		627.8	1,404.0	1,347.7
Current Tax Expense, Domestic												194.6	374.9	414.8
Current State and Local Tax Expense (Benefit)												(0.6)	0.4	0.3
Current Tax Expense, Foreign												33.4	26.0	(29.4)
Income Tax - Current												227.4	401.3	385.7
Deferred Tax Expense (Benefit), Federal												(114.6)	14.2	(14.6)
Deferred State and Local Income Tax Expense (Benefit)												(0.2)	(0.3)	(2.1)
Deferred Tax Expense (Benefit), Foreign												(8.2)	(5.4)	47.3
Income Tax - Deferred												(123.0)	8.5	30.6
Income Tax Expense												$ 104.4	$ 409.8	$ 416.3

[1] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.

[2] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[3] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.